The giant ventral photoreceptors from Limulus (horseshoe crab) provide a model system for the investigation of receptor-mediated signal transduction. In this system, light activates the IP3 transduction pathway. IP3 releases Ca2+ from the endoplasmic reticulum in Limulus photoreceptors. Injection of IP3 or Ca2+ opens ion channels in these cells. In addition, light activates a second pathway of transduction that is not Ca2+-mediated, as shown in experiments with Ca2+-depleted cells. The identity of the channels activated by light are unknown. They may be activated by cGMP and/or Ca2+. A primary goal of this investigation is to sort out the puzzling relations between the IP3 and cGMP pathways in this model system. The specific aims of this project are: (1) to use confocal microscopy to determine the dynamics of local Ca2+ release in response to dim flashes and single photoisomerizations, (2) to determine the mechanisms of phototransduction in Ca2+-depleted photoreceptors, (3) to determine whether Ca2+ activates channels directly and (4) to sequence and localize a cGMP-gated channel in Limulus photoreceptors.